It is relatively easy to re-brand an unlocked 950 Dual Sim into an AT&T 950 if you know what you are doing. But being that there is an ATT rom for the 950 already makes it easier...

I never thought about trying this with a 950 xl only because the easiest way is to just to be able to flash the entire ATT rom to the unlocked 950 after making the initial changes to it. If I have time, I will do more digging on the XL to find if evidence exists of this being impossible with the current resources/knowledge out there. After that I can decide to try, but would need to come across a cheaper 950 xl in order to attempt the aforementioned changes to it.